As near as I can tell, references to built-in messaging on promotion to TL3 are outdated, and those site texts were removed when the Lounge category was removed from the default config. Today you’d need to use Discourse Automation to send a custom welcome message when people reach TL3.
So I believe re-creating a Lounge would take:
- Create a category limited to TL3
- Create automatic PM upon TL3 promotion with a link or reference to the category
- Adjust TL3 settings so it’s not easily lost